A key unit is a type of components constituting mobile equipment such as portable phones and personal digital assistants (PDA), and a unit in which a multiplicity of keys (push buttons) for switch operation are assembled and arrayed on a surface of a single sheet. A single key comprises a key top made of a hard resin adhered to the surface of flexible rubber keypads such as various rubbers including a silicone rubber or thermoplastic elastomers, and a switch-pressing projection (what is called “pressing element”) formed on the back surface of the keypad corresponding to the position of the key top. When a circuit substrate provided with switch elements close contacts with the undersurface of the key unit configured in such manner, key switches are formed at the positions corresponding to each key.
However, the conventional rubber keypads are too soft and lack shape stability, which causes various disadvantages such as incapability of precisely maintaining gaps between key tops and difficulty for applying automatic assembly. In particular, when a casing is provided with a key frame in order to maintain gaps between the keys, such a new disadvantage occurs that the key tops do not return due to interference with the key frame.
In order to overcome these weak points, there is conceived of the configuration of adding a hard plate material (hereinafter, referred to as “reinforcing plate”) to the keypad, so as to impart rigidity to the key unit itself instead of using the key frame which is provided on the casing, and the present applicant has filed a patent application with the title of “hard base key unit” (Patent Reference 1).
Patent Reference 1:
Japanese Unexamined Patent Publication No. 2003-178639
In the hard base key unit, usage of a rubber elastic body is limited only to a key operation part in a keypad, and other parts of the keypad are replaced with a hard resin plate for reinforcement. An aperture is provided at the position corresponding to the key operation part of the hard resin plate which serves as a reinforcing plate, and a rubber elastic body is jointed around the aperture. Further, there are also some alternative conditions that the rubber elastic body covers the whole top surface of the hard resin plate together with the aperture, or a rubber elastic body film enfolds the whole hard resin plate including the apertures.
However, the role of the reinforcing plate in Patent Reference 1 only remains imparting rigidity to the keypads and serving also as a light-conducting plate, and no idea is recognized such that the reinforcing plate is positively utilized for a design element of equipment.